System and method for simultaneously measuring thin film layer thickness, reflectivity, roughness, surface profile and magnetic pattern on thin film magnetic disks and silicon wafers

ABSTRACT

A system and method for performing a magnetic imaging, optical profiling, and measuring lubricant thickness and degradation, carbon wear, carbon thickness, and surface roughness of thin film magnetic disks and silicon wafers at angles that are not substantially Brewster&#39;s angle of the thin film (carbon) protective overcoat is provided. The system and method involve a focused optical light whose polarization can be switched between P or S polarization is incident at an angle to the surface of the thin film magnetic disk. This generates both reflected and scattered light that may be measured to determine various values and properties related to the surface of the disk, including identifying the Kerr-effect in reflected light for determination of point magnetic properties. In addition, the present invention can mark the position of an identified defect.

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/414,388 filed on Oct. 7, 1999, which is acontinuation-in-part of U.S. patent application Ser. No. 09/347,622filed on Jul. 2, 1999 which is a continuation-in-part of U.S. patentapplication Ser. No. 09/136,897 filed on Aug. 19, 1998, now U.S. Pat.No. 6,031,615, which claims priority from provisional application No.60/059,740 filed on Sep. 22, 1997 which are all incorporated byreference herein in their entirety.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is directed toward measuring thin filmsincluding silicon wafers and more particularly toward measuringlubricant thickness, lubricant degradation, thin film thickness andwear, and surface roughness using a laser directed toward a thin filmdisk at many angles including non-Brewster's angles of an absorbinglayer of the thin film.

[0004] 2. Description of Background Art

[0005] Coated thin film disks are used in a variety of industries. Oneexample is the computer hard disk industry. A computer hard disk(magnetic storage device) is a non-volatile memory device that can storelarge amounts of data. One problem that the manufacturers of hard disksexperience is how to maximize the operating life of a hard disk. When ahard disk fails the data stored therein may be difficult, expensive, orimpossible to retrieve.

[0006] A schematic of a thin film disk used in magnetic storage devicesis shown in FIG. 1. It includes a magnetic thin film (layer) 106 whichis deposited upon a substrate 108 (typically a NiP plated Al—Mg alloy orglass). The magnetic thin film 106 can be protected by a thin film ofcarbon 104 (carbon layer), for example, whose thickness is typically 50to 200 Angstroms (Å). The carbon layer 104 is typically coated with athin layer (10 to 30 Angstroms) of a fluorocarbon lubricant 102(lubricant layer). The lubricant layer 102 serves to increase thedurability of the underlying carbon layer 104 particularly when themagnetic read/write head contacts the disk, for example when the diskdrive is turned off, as described below. During the development andtesting of thin film disks it is necessary to subject thin film magneticdisks to numerous starts and stops of the read/write head. Thestart/stops cause the read/write head to contact the thin film disk 100in a dedicated region of the thin film disk 100 known as the start/stopzone. The action of stopping and starting the thin film head on thestart/stop zone can cause depletion and/or degradation of thefluorocarbon lubricant layer 102, wear of the carbon layer 104 andchanges in the surface roughness. A conventional technique for measuringthin film characteristics are discussed in U.S. Pat. No. 4,873,430 whichis incorporated by reference herein in its entirety. This patentdescribes a technique that uses a P polarized collimated (unfocussed)laser propagating at the Brewster's angle of the film to measure filmthickness and surface roughness.

[0007] U.S. Pat. No. 5,726,455 describes an optical system for measuringonly the specular component of light reflected from a thin film magneticdisk. The patent purports that the system is able to measure lubricantcoating thickness and coating wear. This system uses a temperaturestabilized (Peltier effect cooled) light source and an integratingsphere detector which is remotely located from the disk. The angle ofincidence is between the Brewster's angle of the lubricant and that ofthe adjacent layer. This invention makes no provision for themeasurement of the scattered light nor does it measure surfaceroughness.

[0008] Other techniques for measuring surface roughness are discussed inU.S. Pat. Nos. 5,608,527, 5,196,906, 5,313,542, 4,668,860, 5,406,082 andin the book “Optical Scattering-Measurement and Analysis” second editionby John C. Stover, SPIE Press, Bellingham, Wash., 1995 on page 169through 170, which are all incorporated by reference herein in theirentirety. These references relate to obtaining the surface roughness anddo not address identifying lubricant thickness and degradation or thinfilm thickness or wear.

[0009] Specifically, U.S. Pat. No. 5,608,527 describes a technique formeasuring the specular and scattered light in one scattering plane byusing a multi-segmented array. The specular and scattered lights areused to obtain the surface roughness. U.S. Pat. No. 5,196,906 describesa modular scatterometer for determining surface roughness from an arrayof detectors positioned along a hemisphere. U.S. Pat. No. 5,313,542describes a scatterometer which uses depolarized light from a laserdiode and fiber optic bundles to collect partial or full hemisphericallyscattered light. U.S. Pat. No. 4,668,860 describes a scatterometer forevaluating the surface quality of an optical element which has both bulkand surface scatter. This patent describes a technique that purports toseparate surface and bulk scatter by using the polarizationcharacteristics of the light. U.S. Pat. No. 5,406,082 describes asurface inspection and characterization system that uses a broadbandinfrared light source which is directed onto the surface to beinspected. The reflected light is separated into several wavelengths andthese signals are used to characterize the surface by such properties asabsorbency.

[0010] A technique for combining the measurement of thin film thicknessand surface roughness is described in a brochure by AHEADOptoelectronics, Inc., Taipei, Taiwan, R.O.C, which is incorporated byreference herein in its entirety. This describes an instrument called anIntegrating Sphere Ellipsometry Analyzer. This instrument is a combinedellipsometer and integrating sphere analyzer. This brochure teaches ameasurement technique that uses an ellipsometric technique for the exsitu measurement of absolute film thickness and indices of refraction.This technique also uses an integrating sphere to measure surfacemicroroughness at a variable angle. The system as described is designedfor ex situ measurement of film thickness and surface microroughness, itis not capable of measuring in situ wear, lubricant and surfaceroughness.

[0011] A technique for measuring thin film properties at Brewster'sangle is described in S. Meeks et. al., Optical Surface Analysis of theHead-Disk-Interface of Thin Film Disks, ASME Transactions on Tribology,Vol. 117, pp. 112-118, (January 1995), which is incorporated byreference herein in its entirety.

[0012] None of these references teach a single system and method forperforming all of these measurements in situ. In addition, referencesMeeks et al. and Juliana et al. teach that the measurement should occurat substantially Brewster's angle of the carbon 104. U.S. Pat. No.5,726,455 teaches that the measurement should occur between Brewster'sangle of the lubricant and that of the adjacent layer. A stated benefitof using this angle is that the light signal will not reflect off of thecarbon 104 and instead will pass directly through the carbon 104 andreflect off of the magnetic layer 106.

[0013] What is needed is a system and method for examining thin filmdisks that: (1) measures the amount of lubricant thickness and thicknesschange; (2) measures the extent of lubricant degradation; (3) measuresthe wear and thickness of the carbon layer; (4) measures the absolutesurface roughness and changes in the surface roughness; (5) performsmagnetic imaging; (6) performs optical profiling; and (7) enables thesemeasurements to be (a) performed simultaneously, (b) performed at anangle of incidence that is substantially different from Brewster'sangle, and (c) performed in situ or ex situ.

SUMMARY OF THE INVENTION

[0014] The invention is a system and method for measuring thin film diskproperties using an optical system that transmits electromagneticradiation toward the thin film disk at an angle of incidence that neednot be substantially Brewster's angle. The present invention measureslubricant thickness and degradation, carbon wear and thickness, andsurface roughness of thin film magnetic disks at angles that are notsubstantially Brewster's angle of the thin film protective overcoat(carbon). A focused optical light whose polarization can be switchedbetween P or S polarization is incident at an angle to the surface ofthe thin film magnetic disk. This allows the easy measurement of thechange in lubricant thickness due to the interaction of the thin filmhead, the absolute lubricant thickness and degradation of the lubricant.It also allows the measurement of changes in carbon thickness and theabsolute carbon thickness. The surface roughness can also be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is an illustration of a thin film that can be measuredusing the preferred embodiment of the present invention.

[0016]FIG. 2 is an illustration of an apparatus for measuring propertiesof the thin film according to the preferred embodiment of the presentinvention.

[0017]FIG. 3 is a more detailed illustration of a liquid crystalvariable retarder (LCVR) driver according to the preferred embodiment ofthe present invention.

[0018]FIG. 4 is an illustration of the feedback amplification system ofthe preferred embodiment of the present invention.

[0019] FIGS. 5(a)-(c) are illustrations of the reflective and scatteringproperties of P and S polarized radiation according to the preferredembodiment of the present invention.

[0020]FIG. 6 is a more detailed illustration of photodiode electronicsaccording to the preferred embodiment of the present invention.

[0021]FIG. 7 is a flow chart illustrating a method for measuring in situthin film properties according to the preferred embodiment of thepresent invention.

[0022]FIG. 8 is a graph illustrating the reflectance of P and Spolarized radiation versus angle of incidence off a thin film having nolubricant and having ten nanometers of lubricant according to thepreferred embodiment of the present invention.

[0023]FIG. 9 is a graph illustrating the reflectance of P and Spolarized radiation versus angle of incidence off a thin film havingtwenty nanometers of carbon and having fifteen nanometers of carbonaccording to the preferred embodiment of the present invention.

[0024]FIG. 10 is a two dimensional concentration histogram illustratingthe relationship between changes in S polarized radiation and Ppolarized radiation with respect to thin film measurements when an angleof incidence of the radiation source is between 53 degrees and 71degrees according to one embodiment of the present invention.

[0025]FIG. 11 is a two dimensional concentration histogram illustratingthe relationship between changes in S polarized radiation and Ppolarized radiation with respect to thin film measurements when an angleof incidence of the radiation source is approximately 53 degreesaccording to one embodiment of the present invention.

[0026]FIG. 12 is a two dimensional concentration histogram illustratingthe relationship between changes in S polarized radiation and Ppolarized radiation with respect to thin film measurements when an angleof incidence of the radiation source is less than 53 degrees accordingto one embodiment of the present invention.

[0027]FIG. 13 is a two dimensional concentration histogram illustratingthe relationship between changes in S polarized radiation and Ppolarized radiation with respect to thin film measurements when an angleof incidence of the radiation source is between 71 degrees and 90degrees according to one embodiment of the present invention.

[0028]FIG. 14 is a theoretical graph illustrating the change in Pspecular reflectivity verses the thickness of a carbon layer innanometers (nm).

[0029]FIG. 15 is a graph illustrating the sensitivity of P polarizedlight reflectivity to carbon wear verses the k of carbon for a lightsignal having a wavelength of 650 nm and having an angle of incidence of58 degrees.

[0030]FIG. 16 is an illustration of a two dimensional fast Fouriertransform of a S specular image as measured by the preferred embodimentof the present invention.

[0031]FIG. 17 is an illustration of a cut through the fast Fouriertransform showing the texture angles, width and texture amplitudedistribution of a disk texture line pattern.

[0032]FIG. 18 is a flow chart illustrating a method for measuring carbonwear for an in situ process according to an embodiment of the presentinvention.

[0033]FIG. 19 is a flow chart illustrating a method for measuring carbonwear for an ex situ process according to an embodiment of the presentinvention.

[0034]FIG. 20 is an illustration of a high temperature thin filmmeasurement system 2000 according to one embodiment of the presentinvention.

[0035]FIG. 21 is an illustration of a computer system according to anembodiment of the present invention.

[0036]FIG. 22 is a flowchart illustrating the operation of the symmetryunit 2112 according to one embodiment of the present invention.

[0037]FIG. 23 is a flow chart illustrating the operation of thehistogram subtraction unit 2114 according to one embodiment of thepresent invention.

[0038]FIG. 24 is a flow chart illustrating the operation of the AND/NOTunit 2116 according to one embodiment of the present invention.

[0039]FIG. 25 is an example of a simplified two-dimensional (2D)histogram image according to an embodiment of the present invention.

[0040]FIG. 26 is a chart illustrating an analysis technique according toone embodiment of the present invention.

[0041]FIG. 27 illustrates one example of histogram analysis according tothe AND/NOT unit of the present invention.

[0042]FIG. 28 illustrates one example of histogram analysis using Pspecular or S specular versus P scattered or S scattered variableaccording to the AND/NOT unit of the present invention.

[0043]FIG. 29 illustrates the preferred embodiment of the apparatus formeasuring the properties of thin films including thickness,reflectivity, roughness, magnetic pattern and surface profile.

[0044]FIG. 30 illustrates the preferred embodiment of the apparatus formeasuring the properties of thin films and the ability to scribedefects.

[0045]FIG. 31 illustrates the interpretation of the phase shift data ina two-dimensional concentration histogram of P_(Q) versus S_(Q) for 45degree linearly polarized light.

[0046]FIG. 32 illustrates a schematic representation of erosion anddishing on a CMP polished silicon wafer.

[0047]FIG. 33 illustrates the actual measurement of phase shift(proportional to film thickness) versus position on a die on a patternedsilicon wafer. The figure also illustrates the definitions of dishingand erosion.

[0048]FIG. 34 illustrates the various quadrants of a quad-cell detectorused for circumferential and radial optical profiles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] A preferred embodiment of the present invention is now describedwith reference to the figures where like reference numbers indicateidentical or functionally similar elements. Also in the figures, theleft most digit(s) of each reference number correspond(s) to the figurein which the reference number is first used.

[0050]FIG. 2 is an illustration of an apparatus for measuring propertiesof the thin film according to the preferred embodiment of the presentinvention. The apparatus uses a focused laser light signal whose angleof propagation can be between zero degrees from normal and ninetydegrees from normal.

[0051] One embodiment of the apparatus 200 includes a conventional laserdiode 202, e.g., SLD 104AU available from Sony, Tokyo, Japan, which hasbeen collimated by Hoetron Corp., Sunnyvale, Calif., e.g., aconventional linear polarizer 204, e.g., made of Polarcor that iscommercially available from Newport Corp., Irvine, Calif., aconventional liquid crystal variable retarder 206 that is commerciallyavailable from Meadowlark, Longmont, Colo., a conventionalnon-polarizing beam splitter 208 that is commercially available fromNewport Corp., Irvine, Calif., a conventional diffuser 210 that iscommercially available from Spindler Hoyer, Germany, a conventionalfeedback photodiode 212 that is commercially available from HamamatsuCorp., Hamamatsu City, Japan, a feedback amplifier and receiverpreamplifier 214 that is commercially available from Analog Design, SanFrancisco, Calif., a conventional focusing lens 216 that is commerciallyavailable from Newport Corporation, Irvine, Calif., a custom integratingsphere 218 that is commercially available from Labsphere, North Sutton,N.H., made from an approximately 0.62 inch cube aluminum block, whichhas a 0.54 inch diameter spherical section removed from its center, a 4mm diameter hole at the bottom for the scattered light to enter thesphere, another 4 mm diameter hole at the top for the light to reach aphotodetector, two holes at opposite sides for the specular light toenter and exit the sphere, the internal surfaces are coated with areflective surface that scatter light, e.g. Spectralflect, availablefrom Labsphere, North Sutton, N.H. The integrating sphere 218 has aninput hole that is designed to be slightly larger than the laser beamdiameter so as not to occlude the beam, the output hole diameter ischosen to be large enough to allow the beam to exit the sphere and smallenough to allow for the detection of the minimum spatial frequencyaccording to equation (3), the diameter of the holes and the diameter ofthe sphere are chosen so that the total surface area of the holes ispreferably less than five percent of the total surface area of thesphere, although the use of a larger percentage is possible. Theintegrating sphere 218 preferably has a baffle extending through itscenter in the same plane as the incidence light, the baffle has acircular region at its center which is the same diameter as the holeprovided for the scattered photodetector. The baffle prevents any firstreflection from the disk from reaching the photodetector without firststriking the surface of the integrating sphere. The integrating sphere218 is preferably miniaturized to keep the entire optical device small.

[0052] One embodiment of the apparatus 200 also includes a conventionalcollimating lens 220 that is commercially available from NewportCorporation, Irvine, Calif., a conventional diffuser 222, that iscommercially available from Spindler Hoyer, Germany, a conventionalspecular photodetector 224A and scattered photodetector 224B that iscommercially available from Hamamatsu Corp., Hamamatsu City, Japan, acustom baffle 226 that is commercially available from Labsphere, NorthSutton, New Hampshire, a liquid crystal variable retarder (LCVR) driver228, and a conventional computer 240, for example a microcontroller oran IBM personal computer, commercially available from IBM Corporation,Armonk, N.Y. having a conventional input/output device 242, aconventional memory module 244 having unconventional applications storedtherein, e.g., the analysis unit 248, and a conventional processor 246,e.g., a Pentium Pro Processor that is commercially available from IntelCorporation, Santa Clara, Calif. It will be apparent to persons skilledin the art that the apparatus 200 is the preferred embodiment of thepresent invention and that alternate designs can be used withoutdeparting from the present invention. The operation of the apparatus 200is now described in greater detail.

[0053] A laser diode 202 emits an electromagnetic signal toward the thinfilm disk. In the preferred embodiment the electromagnetic signal is alight signal having a wavelength of 780 nanometers (nm) although a widevariety of wavelengths can be used. The angle of propagation of thelight signal can be any angle between zero and ninety degrees. However,in the preferred embodiment the angle need not be substantiallyBrewster's angle for the carbon in the thin film. That is, the angle ofpropagation differs from the Brewster's angle of the carbon by a minimumof two to five degrees, for example, at which angle the change in thereflectivity of the thin film based upon the carbon changessignificantly when compared to the reflectivity at Brewster's angle. Theemitted light passes through the linear polarizer 204. The linearpolarizer 204 improves the linear polarization of the laser lightsignal. The polarized light signal passes through a liquid crystalvariable retarder (LCVR) 206. The LCVR 206 switches the polarization ofthe light between P and S linear polarizations in response to aninstruction received from the LCVR driver 228. The LCVR driver 228 canbe located external to or integral with the computer 240. As describedbelow, P and S linear polarizations enable the apparatus 200 to measurea variety of properties of the thin film 100. A description of oneexample of the LCVR driver is now described with reference to FIG. 3.

[0054]FIG. 3 is a more detailed illustration of the LCVR driver 228according to the preferred embodiment of the present invention. The LCVRdriver 228 includes an amplitude control module 302, a gaussian noisemodule 304, a crystal oscillator 306, and a low pass filter 308. In thepreferred embodiment the crystal oscillator is a 2 kHz square waveoscillator whose fundamental frequency is modulated by five percent in arandom manner by Gaussian noise generated by the gaussian noise module304. The 2 kHz square wave amplitude is controlled in two states by theamplitude control module 302 that receives signals from the computer 240so that the P and the S polarizations are achievable. The output of theoscillator is low pass filtered by the low pass filter 308 having acutoff at approximately 15 kHz before being directed to the liquidcrystal. The random modulation of the square wave helps preventcrosstalk in the apparatus 200 from being synchronous with the datasampling.

[0055] The linear polarized signal is received by the non-polarizingbeam splitter 208 that splits the linear polarized signal. A portion ofthe linear polarized signal is split and is directed toward a diffuser210 and to a feedback photodetector 212. The output of the feedbackphotodetector 212 is received by a feedback amplifier in the feedbackamplifier and receiver preamplifier 214. FIG. 4 is an illustration ofthe feedback amplification system of the preferred embodiment of thepresent invention. The feedback amplification system of the preferredembodiment includes a negative feedback amplifier 402 that receives theoutput of the feedback photodetector 212. The negative feedbackamplifier 402 outputs a signal to the laser diode that preciselycontrols the intensity of the laser diode 202. In one embodiment of thepresent invention the bandwidth of the feedback loop is limited to 15Hz. This allows stablilization of the laser power between DC and 15 Hz.The bandwidth of the feedback loop is sharply cut off above 15 Hz toprevent power frequencies (60 Hz and its harmonics) from modulating thelaser power. An advantage of the external beam splitter 208 togetherwith the reference photodiode 212 is improved temperature stability. Theimproved temperature stability is achieved since the referencephotodiode 212 is identical to the specular 224A and the scattered 224Bphotodetectors. Any temperature changes in the optical sensitivity ofthe reference photodiode 212 are substantially compensated by similarchanges in the specular 224A and scattered 224B photodetectors.

[0056] Laser diodes are well known to have an internal photodiode tomonitor the laser output power. Another embodiment of a feedback controlcircuit to control the optical intensity is to use such a photodiode,which is internal to the laser diode. This laser diode feeds back acontrol signal to the circuitry described in FIG. 4 and by doing sokeeps the intensity of the laser at a constant value.

[0057] Conventional systems, for example those described in Meeks, etal., Optical Surface Analysis of the Head-Disk-Interface of Thin FilmDisks, ASME Transactions on Tribology, Vol. 117, pp. 112-118, (January1995)) describe the use of narrow band pass filters (NBPF) on thephotodetectors in order to minimize interference from external lightsources. The NBPF allows only the specified wavelength to reach thedetector. One drawback of this method is that it requires the laser tobe stable at the specified wavelength. This is difficult as the laserwavelength is affected by temperature change, thus the system has to bethermally stabilized.

[0058] To eliminate the effect of external light on the instrument, theentire device 200 is enclosed in a light tight container whicheliminates the possibility of external light from reaching thedetectors. As a result the NBPF can be eliminated from the design.Removing the NBPF greatly reduces the effect of temperature changes onthe signal amplitude. This improves the thermal stability of the system.

[0059] Another means to improve the stability of the system is to removeelectronic zero drift by use of a black standard. A black standard is adevice that absorbs any light coming toward it. One version of a blackstandard is a cylindrical cavity with a pointed cone inside thecylinder, with all the internal surfaces coated with a black, lightabsorbing material. This is type of black standard is commerciallyavailable from Labsphere, North Sutton, N.H. The electronics typicallydrift over time due to thermal changes, component age, and otherfactors. The black standard provides a stable zero level reference tomeasure and cancel the drift. Prior to each scan the laser beam isdirected into the black standard, the electronics signals are thenmeasured and the zero level is defined. This results in improvedstability in the long-term drift of the zero levels of the system.

[0060] The linearly polarized signal that passes through thenon-polarizing beam splitter 208 is directed toward a focusing lens 216that focuses the light signal onto an area of the thin film 100 that islocated beneath the integrating sphere 218 (a cross-sectional view ofthe integrating sphere is illustrated in FIG. 2). A first portion of thefocused light signal reflects off the thin film 100 toward a collimatinglens 220 and a second portion scatters within the integrating sphere218. A more detailed discussion of the reflecting and scattering of thefocused light signal is now set forth.

[0061] FIGS. 5(a)-(c) are illustrations of the reflective and scatteringproperties of P and S polarized radiation according to the preferredembodiment of the present invention. The view of FIGS. 5(a)-(c) are froma reverse-angle view in comparison to the view of FIG. 2. FIG. 5(a)illustrates the reflection of the focused light signal (P polarizedlight) off the thin film 100. As described above, the focused Ppolarized light signal is directed toward the thin film 100 at an angle,e.g., an angle that is not substantially Brewster's angle. Some of thefocused P polarized light signal reflects off the lubricant layer 102.Some of the focused P polarized light signal reflects off the carbonlayer 104 while some of the focused P polarized light is absorbed by thecarbon layer, and some of the P polarized light reflects off themagnetic layer 106. FIG. 5(b) illustrates the reflection of the focusedlight signal (S polarized light) off the thin film 100. As describedabove, the focused S polarized light signal is also directed toward thethin film 100 at an angle that is not substantially Brewster's angle.The reflection of the S polarized light is similar to the reflection ofthe P polarized light described above. Specifically, some of the focusedS polarized light signal reflects off the lubricant layer 102. Some ofthe focused S polarized light signals reflect off the carbon layer 104while some of the focused S polarized light is absorbed by the carbonlayer, and some of the S polarized light reflects off the magnetic layer106.

[0062] The reflected (specular) light signals I_(sp) pass through anopening in the integrating sphere 218 toward an optional collimatinglens 220. The collimating lens collimates the reflected light signalswhich enables the diffuser 222 and specular photodetector 224A to bepositioned at a further distance from the reflection area on the thinfilm disk 100 than would otherwise be possible. The diffuser spreads thebeam in a manner such that the position sensitivity of the specularphotodetector is reduced. This reduces the sensitivity of thephotodetector to motion of the optical beam induced by wobble of thedisk. The diffuser 222 and the specular photodetector 224A arepositioned at an angle that is slightly off the normal (e.g., fivedegrees) of the reflected light path. This geometry reduces the amountof light signals that are reflected off of the diffuser 222 and/or thespecular photodetector 224A and propagate back into the integratingsphere which can possibly affect the detection of scattered light, asdescribed below. That is, the addition of the collimating lens 220 whichcollimates the light allows the path length to be increased so that theamount of tilt of the specular photodetector 224A and the diffuser 222is minimized. When the amount of tilt of the specular photodetector 224Aand diffuser 222 is reduced, the specular photodetector will receive agreater portion of the reflected signal since the amount of the specularsignal lost due to a reflection off the diffuser 222 or the specularphotodetector 224A is minimal. In the preferred embodiment, thecollimating lens 220 is used for a high resolution (short focal length)design. A lower resolution system (longer focal length lens) generallyallows sufficient length between the specular photodiode and theintegrating sphere to require only a small tilt of the specularphotodiode. The specular signal must not be allowed to return to theintegrating sphere since this corrupts the scattered signal and causes acrosstalk between the scattered and specular signals. The diameter ofthe light port in the optic main body is kept to a minimum to block mostof the light that is reflected by any surface in the specular detectorarea toward the integrating sphere. The port diameters are made justslightly larger than the beam diameter itself. They can be stepped (notcontinuously tapered) to make it easier to fabricate. The specularphotodetector 224A outputs a signal representing the amount of lightreceived to the receiver preamplifiers in the feedback amplifier andreceiver preamplifiers board 214. The received light is interpretedusing the computer 240 in the manner described below. The operation ofthe specular photodetector 224A is described in greater detail below.

[0063]FIG. 5(c) illustrates the scattering effect of the S or Ppolarized light signals. When the focused light signal strikes thelubricant layer 102, the carbon layer 104, and/or the magnetic layer106, a portion of the light will scatter at angles that are not equal tothe angle of incidence. For simplicity, FIG. 5(c) only illustratesreflection off the lubricant layer 102. The scattered component of thelight is measured by the scattered photodetector 224B attached to theintegrating sphere 218. Internal to the integrating sphere 218 is abaffle 226 which does not permit any first reflection scattered light toreach the scattered photodetector 224B. This baffle 218 reduces themeasurement of hot spots caused by a direct reflection from the diskinto the scattered photodetector 224B. The baffle 218 prevents this byforcing any reflections from the thin film disk 100 to take two or morereflections before reaching the scattered photodetector 224B.

[0064] As described above, the LCVR 206 allows the polarization to beswitched between P and S linear polarizations. The P specular lightsignal primarily gives information regarding changes in the thickness,or the absolute thickness of the carbon layer on the thin film disk. TheS specular light signal primarily gives information regarding changes inthe lubricant thickness which has been applied to the carbon surface.The scattered light, together with the specular light gives ameasurement of the roughness of the thin film disk surface. The methodfor using the specular and scattered components of the P and S polarizedlight to measure thin film 100 characteristics is described below.

[0065]FIG. 6 is a more detailed illustration of a photodetector 224according to the preferred embodiment of the present invention. Thephotodetector can be the specular photodetector 224A or the scatteredphotodetector 224B. The photodetector 224 of the preferred embodimentincludes a biased photodiode 602, a transconductance preamplifier 604, abuffer amplifier 606, signal conditioning circuitry 607 (available fromAnalog Design, Inc. in Topanga, Calif.) and a GAGE Applied Sciences,Inc. analog to digital board 608, e.g., model number CS1012/PCI that iscommercially available from GAGE Applied Science, Inc., Montreal,Canada. The biased photoconductor 602 receives a light signal andgenerates a signal reflecting the intensity of the received light. Thebiased photodiode signal is amplified by the transconductancepreamplifier 604 which is transmitted to the buffer amplifier 606.Before being digitized by the analog to digital board the signal passesthrough signal conditioning electronics 607. The signal conditioningelectronics 607 subtracts the DC offset of the signal, passes the signalthrough a variable anti-aliasing filter and provides up to 64 timesmultiplication of the encoder signal from the spindle which rotates thethin film disk 100, in the preferred embodiment. The multiplied encodersignal and the index from the spindle are used as the clock and trigger,respectively for the analog to digital board. After the specular andscattered signals have been conditioned they are passed to the analog todigital board 608 where they are digitized. The digitized signal istransmitted to the computer 240 for analysis. The method for analyzingthe received signals to determine properties of the thin film disk 100is set forth below.

[0066] The properties of the entire thin film disk 100 can be measuredby focusing the light signal on all areas of the thin film disk 100.This can be accomplished by precisely moving the thin film disk 100 orby moving the apparatus 200. In the preferred embodiment the apparatus200 is attached to a very accurate stepper motor (not shown) and theapparatus 200 is stepped over the surface of the thin film disk 100. Oneexample of such a stepper motor is Newport's Mikroprecision stage thatis commercially available from Newport, Irvine, Calif.

[0067]FIG. 7 is a flow chart illustrating a method for measuring thinfilm properties according to the preferred embodiment of the presentinvention. In the preferred embodiment a differential technique is usedsuch that reference images of the thin film disk are made at thebeginning of the experiment and the reference images are subtracted fromeach of the subsequent images. The resulting differential images showonly what has changed as a result of interacting with the disk duringthe period of time between the reference and subsequent images. Thereference images are not a requirement to analyze the data but it makesany changes in the disk surface easier to identify and increases thesensitivity to changes. However, in alternate embodiments only a singleset (Ssp, Ssc, Psp, and Psc) of images is measured (at a angle that isnot substantially Brewster's angle for the carbon layer 104, forexample) and lubricant thickness and degradation, carbon wear andsurface roughness is determined.

[0068] The apparatus 200 measures 702 reference values of the thin filmdisk at an angle that is not substantially Brewster's angle (asdescribed above). These reference values include the specular andscattered components of the P and S polarized signals received by thespecular photodetector 224A and the scattered photodetector 224B,respectively. The measurements can be taken in situ or ex situ, asdescribed below. The user then performs 704 an action on the thin filmdisk 100. For example, the thin film disk 100 is subjected to repeatedstart-stop actions such that a ceramic slider of the read/write headrepeatedly contacts the thin film disk 100. This simulates repeatedpower on/off cycles of a hard disk drive, for example. This contact cancause lubricant depletion, lubricant degradation, and carbon layer wear.After the first iteration of action is performed on the thin film disk100, e.g. a thousand start/stop simulations, the apparatus measures 706new values of the thin film disk 100. These new values include thespecular and scattered components of the P and S polarized signalsreceived by the specular photodetector 224A and the scatteredphotodetector 224B, respectively. The signals representing these valuesare stored in the computer memory module 244 and an analysis unit 248 inthe memory module analyzes the values in conjunction with the processor246. The finctions performed by the analysis unit 248 are describedbelow.

[0069] The analysis unit 248 determines 708 differential values bydetermining the difference in values between each reference value andthe corresponding value in the subsequent measurement of the thin filmdisk 100. The differential values include the difference (delta) in thespecular component of the S polarized light (ΔS_(SP)), i.e., thereflectance received by the specular photodetector 224A when S polarizedlight is transmitted toward the thin film disk, the delta in thespecular component of the P polarized light (ΔP_(SP)), the delta in thescattered component of the S polarized light (ΔS_(SC)), and the delta inthe scattered component of the P polarized light (ΔP_(SC)). Thesedifferential values are used to identify thin film properties asdescribed below. One technique for measuring the reflectance of a lasersignal striking the thin film disk 100 at Brewster's angle of the carbonlayer 104 is described in the S. Meeks et. al., Optical Surface Analysisof the Head-Disk-Interface of Thin Film Disks, ASME Transactions onTribology, Vol. 117, pp. 112-118, (January 1995), that was incorporatedby reference above.

[0070] The subtraction of the reference and subsequent images isdegraded by the presence of thermal drift during the time between thegathering of the reference and subsequent images. This thermal drift iscaused by the thermal expansion of the disk and other components withvariations in environmental temperature. The thermal drift can becorrected by shifting each image with respect to the other in the radialdirection of the thin film disk. The shifted images are shifted and thecross correlation between the two images is computed. The amount ofshift is increased and the cross correlation is repeated until a maximumis reached. The shift at which the maximum in the cross correlationoccurs is the optimal shift, i.e., the one which corrects for thethermal drift of the components. An alternative to using the crosscorrelation is to subtract the images and compute the variance orstandard deviation between the two images. The shift is then increasedand the variance or standard deviation is again computed. The amount ofshift which minimizes the standard deviation or variance is the optimalshift which will correct for the thermal drift.

[0071] In order to better understand the method for analyzing thedifferential values, a description of the effect of the thickness of thelubricant layer and the thickness of the carbon layer 104 on the amountof light received by the specular photodetector 224A and the scatteredphotodetector 224B is now set forth.

[0072]FIG. 8 is a graph illustrating the reflectance of P and Spolarized radiation off a thin film having no lubricant layer 102 andhaving a lubricant layer 102 whose depth is ten nanometers according tothe preferred embodiment of the present invention. FIG. 8 shows thesimulated specular reflectivity of the S and P polarized light versusthe angle of incidence of the light signal on the thin film disk. Inthis example the light signal has a wavelength of 650 nm. Two curves areshown, one with no lubricant applied (black) to the carbon surface andthe other with ten mn of lubricant (white) applied. An unrealisticallythick layer of lubricant has been shown in this figure to illustrate thedifferences between the curves. The difference between the two curvesrepresents the P and S polarized specular light sensitivity tolubricant. At angles between zero degrees and approximately 53 degreesthe reflectivity of the disk decreases when lubricant is added to thedisk for both the P and S polarized light signals. At angles aboveapproximately 53 degrees the reflectivity of the disk decreases for Spolarized light and increases for P polarized light, when lubricant isadded to the carbon surface. At approximately 53 degrees, the Ppolarized light is insensitive to lubricant on the surface, because thisis the Brewster's angle of the lubricant. At angles near 80 degrees theP polarized light reaches a maximum in its sensitivity tolubricant—approximately 2 or 3 times the sensitivity of the S polarizedlight. The angle of 53 degrees is a specific example of the Brewster'sangle of the lubricant which is defined as the Arc Tan of [index ofrefraction of lubricant/index of refraction of air].

[0073] The ratio between the P sensitivity to lubricant and the Ssensitivity changes as a function of the angle as can be seen in FIG. 8.At a fixed angle of incidence this ratio is related to the index ofrefraction of the lubricant. Therefore, if the lubricant degrades, theindex of refraction also changes and the ratio of the change in thespecular component of the S polarized light (ΔS_(SP)) to the change inthe specular component of the P polarized light (ΔP_(SP)) will change.This is one technique for measuring the degradation of the lubricant ona thin film disk 100. The angle of 53 degrees is particularly good forthis since even a very small change in the lubricant index will generatea large change in the ratio of delta S (ΔS_(SP)) to delta P (ΔP_(SP)).

[0074]FIG. 9 is a graph illustrating the reflectance of P and Spolarized radiation off a thin film having a carbon layer 104 of twentynanometers and having a carbon layer of fifteen nanometers according tothe preferred embodiment of the present invention. The black curve showsthe S and P reflectivity versus angle of incidence with 20 nm of carbonpresent. The white curve shows the same curves when 5 nm of carbon hasbeen removed. Both the S and P polarization's can be used to measurecarbon wear, but in general P is more sensitive and more linear in itsresponse to wear of the carbon. The S reflectivity increases when carbonis removed at all angles of incidence. The P polarized light increasesfor carbon removal for angles less than approximately 71 degrees, iszero at approximately 71 degrees and decreases when the angle is greaterthan 71 degrees. The maximum sensitivity to carbon thickness or carbonwear occurs near zero degrees. The angle of 71 degrees is a specificexample of the “P polarization crosspoint angle” which is defined as theangle at which the P polarized beam reflection coefficient isinsensitive to the carbon thickness change.

[0075] In the preferred embodiment the angle of incidence is 58 degreesto measure all lubricant features, carbon thickness and wear and surfaceroughness. However, as described above, many angles can be used.Operation at 58 degrees allows the user to easily separate lubricantthickness increase (P reflectivity increase, S decrease, see FIG. 8)from carbon wear (S and P reflectivity increase). This technique formeasuring carbon wear is not limited to carbon overcoats. The wear ofany absorbing layer can be measured by the embodiments discussed here.In particular, overcoats such as Zirconium Oxide, Silicon Dioxide,organic materials and plastics, for example, can be used. If thesealternative overcoats have lubricant on them, then the lubricantthickness, depletion and degradation may be measured as well.

[0076] One technique for identifying the lubricant thickness change fromcarbon wear based upon ΔS_(SP) and ΔP_(SP) is to use a two-dimensionalconcentration histogram. A technique for generating and using atwo-dimensional concentration histogram is described by S. Meeks et.al., Optical Surface Analysis of the Head-Disk-Interface of Thin FilmDisks, ASME Transactions on Tribology, Vol. 117, pp. 112-118, (January1995), that was incorporated by reference above. To construct a twodimensional histogram small regions (known as buckets) are defined inthe P, S plane (the space of the histogram) which have a certain ΔP byΔS dimension. Each coordinate pair (x,y) in the real space image isselected and its corresponding bucket into which its P, S coordinatefalls is identified. After going through the entire image the totalnumber of points in each bucket is identified and a color, for example,is assigned based upon the number of points in the bucket. The completedtwo dimensional image is known as a two-dimensional concentrationhistogram. The two-dimensional concentration histogram separates theregions of lubricant pooling, depletion, carbon wear and debris intoseparate regions. Debris are products generated as a result of the wearprocess on the disk. In addition, the slope of the histogram is relatedto the index of refraction of the layer being altered. As describedbelow, the slope of the histogram can be used to differentiate betweenlubricant depletion and carbon depletion. Some examples of histogramswhich each corresponds to a different embodiment of the presentinvention are set forth in FIGS. 10-13.

[0077]FIG. 10 is a histogram illustrating the relationship betweenchanges in S polarized radiation (ΔS_(SP)) and P polarized radiation(ΔP_(SP)) with respect to thin film measurements when an angle ofincidence of the radiation source is between approximately 53 degreesand approximately 71 degrees according to the preferred embodiment ofthe present invention. The angle of 53 degrees is a specific example ofthe Brewster's angle of the lubricant which is defined as the Arc Tan of(index of refraction of lubricant/index of refraction of air). The angleof 71 degrees is a specific example of the “P polarization crosspointangle” which is defined as the angle at which the P polarized beamreflection coefficient is insensitive to the carbon thickness change.

[0078] In the preferred embodiment, the analysis unit 248 identifies 710lubricant pooling or depletion or identifies 712 carbon wear using thedifferential specular values (ΔS_(SP), ΔP_(SP) ) in the followingmanner. When the angle of incidence that the focused light signalstrikes the thin film is between approximately 53 degrees and 71degrees, if the value of ΔS_(SP) is positive and the value of ΔP_(SP) isnegative then the analysis unit 248 determines that thin film disk 100has experienced lubricant depletion. Using the histogram illustrated inFIG. 10 this is determined by locating the quadrant in which theΔS_(SP), ΔP_(SP) data is located, in this example, the data is locatedin quadrant II which is identified as lubricant depletion. The analysisunit 248 determines the amount of lubricant depletion or pooling basedupon the value of ΔS_(SP) and a calibration of the amount of ΔS_(SP)change per angstrom of lubricant change.

[0079] This range of angle of incidence allows easy distinction in themeasurements of lubricant pooling/depletion, carbon wear, and debris.The lubricant pooling, depletion, carbon wear, and debris will be indifferent quadrants of the two dimensional histogram, making it easierto separate the data. This data from each of the four quadrants can betraced back to the original images (P and S images in real space)indicating the locations on the disk of lubricant pooling, depletion,carbon wear, and debris.

[0080] If both ΔS_(SP) and ΔP_(SP) are positive then the analysis unit248 determines that the thin film disk 100 experienced carbon wear. Theanalysis unit can determine the amount of carbon wear using a variety oftechniques. Some of these techniques are described below. If ΔS_(SP) isnegative and ΔP_(SP) is positive then the analysis unit 248 determinesthat the thin film disk 100 experienced lubricant pooling. As describedabove, the ratio of ΔS_(SP)/ΔP_(SP) correlates to the index ofrefraction of the lubricant. The expected value of this ratio is knownand is stored in the computer memory module 244. If the analysis unit248 determines that the ratio of ΔS_(SP)/ΔP_(SP) does not correspond tothe expected value of this ratio, the analysis unit determines 714 thatlubricant of the lubricant layer degraded.

[0081]FIG. 11 is a histogram illustrating the relationship betweenchanges in S polarized radiation (ΔS_(SP)) and P polarized radiation(ΔP_(SP)) with respect to thin film measurements when an angle ofincidence of the radiation source is approximately 53 degrees (ingeneral, Brewster's angle of the lubricant) according to one embodimentof the present invention. This angle of incidence enhances sensitivityto changes in the lubricant index of refraction. This is an embodimentwhich is optimized for measuring lubricant degradation. A change in thelubricant index of refraction is related to the degradation of thelubricant.

[0082]FIG. 12 is a histogram illustrating the relationship betweenchanges in S polarized radiation (ΔS_(SP)) and P polarized radiation(ΔP_(SP)) with respect to thin film measurements when an angle ofincidence of the radiation source is less than 53 degrees according toone embodiment of the present invention. This range of angle ofincidence enhances sensitivity to lubricant, carbon thickness change andabsolute carbon thickness. This embodiment is optimized for measuringlubricant pooling/depletion, carbon wear and carbon thickness.

[0083]FIG. 13 is a histogram illustrating the relationship betweenchanges in S polarized radiation (ΔS_(SP)) and P polarized radiation(ΔP_(SP)) with respect to thin film measurements when an angle ofincidence of the radiation source is between 71 degrees and 90 degreesaccording to the preferred embodiment of the present invention. Thisrange of angle of incidence has the highest sensitivity to lubricantthickness change, specifically the P-polarized light. This embodiment isoptimized for measuring lubricant pooling/depletion. When in this rangeof angles, the spatial frequency of the measured surface roughness isnearly twice as large as when measured at near normal incidence. Thisallows the measurement of high spatial frequency roughness(microroughness).

[0084] The technique used for analyzing these histograms is similar tothe description set forth above. With respect to FIG. 12, since thevalues of ΔS_(SP) and ΔP_(SP) are both positive for lubricant depletionand carbon wear, one technique for identifying which occurs is todetermine the point at which the slope of the histogram changes, asillustrated in FIG. 12. The ΔP_(SP), ΔS_(SP) histograms are constructedby subtracting the reference images (taken before any testing has begun)from data gathered during the testing procedure (start/stops, thin filmhead flying or dragging). The differential images are constructed asdescribed earlier and the analysis described above is applied to thehistograms. A time sequence of histograms can be constructed bysubtracting images at various time points from the reference images. Inthis manner the evolution of the histograms and hence the disk surfacecan be followed and analyzed.

[0085] The analysis unit 248 identifies 716 the surface roughness of thethin film disk 100. The roughness is measured simultaneously with themeasurement of the specular light and the scattered light. The analysisunit 248 uses the equation (1) to determine the RMS (root means square)roughness of the thin film disk 100. $\begin{matrix}{{{RMS}\quad {roughness}} = {R_{Q} = \frac{\left\lbrack {({TIS})^{1/2}*\lambda} \right\rbrack}{4*\pi*{\cos (\phi)}}}} & (1)\end{matrix}$

[0086] Where λ is the wavelength of the light signal, φ is the angle ofincidence of the light signal, and TIS is the total integrated scatteredportion of the light signal and is defined by equation (2).$\begin{matrix}{{TIS} = \frac{SC}{{SP} + {SC}}} & (2)\end{matrix}$

[0087] Where SC is the total scattered light and SP is the totalspecular light. As indicated above, the wavelength in the above equationis the wavelength of the incident light. In the preferred embodiment thewavelength is either 780 nm or 650 nm, but in alternate embodiments thewavelength can be any visible or invisible light wavelength. The maximumspatial frequency over which the roughness is measured is determined bythe wavelength of light and the angle of incidence according to equation(3), for example.

f _(g)=(sin(φ₁)−sin(φ_(i)))/λ  (3)

[0088] Where f_(g) is the maximum spatial frequency, φ₁ is the maximumscattering angle and φ_(i) is the angle of incidence. The minimumspatial frequency is determined by the spot size or the exiting hole ofthe sphere, whichever yields a higher number.

[0089] The measurement of the scattered light is used to measure theamount of carbon wear and the carbon thickness. The incident intensityis given in equation (4). Equation (4) is simply a statement of theconservation of energy.

I _(i) =I _(A)+(I _(SP) +I _(SC))  (4)

[0090]

[0091] Where I_(i) is the incident intensity and I_(A) is the absorbedintensity, which is related to the wear of the carbon film and thethickness of the carbon, I_(sp) is the specularly reflected intensityand I_(SC) is the scattered intensity. The incident intensity is keptfixed and in order to measure the absorbed intensity and hence thecarbon wear or carbon thickness it is necessary to measure both thespecular and scattered light at the given angle of incidence. Thealgorithm for measuring carbon wear can be separated into two cases. Thefirst case is known as an in situ wear measurement described above. FIG.18 shows the flow chart for determining carbon wear for the in situcase. The process includes placing a disk within a test stand and taking1802 reference images at the very beginning of the experiment. Thereference images are the S_(SP), S_(SC) and P_(SP), and P_(SC) imagesbefore anything has been done to the surface of the disk. The disk isthen subjected 1804 to start/stop actions of a thin film magnetic heador any other process which might cause wear of the carbon protectiveovercoat. Intermediate to the start/stops, the disk is scanned 1806numerous times to follow the wear process. At the completion of theexperiment the differential images are constructed 1808 by subtractingthe reference images from the images taken before the start/stop actionsupon the disk. The two dimensional concentration histograms areconstructed by summing ΔP_(SP) and ΔP_(SC) (the difference images formedby subtracting the reference image from the intermediate image) andmaking the two-dimensional concentration histogram with thecorresponding ΔS_(SP) summed with ΔS_(SC). The images can be low passfiltered 1810 and high pass filtered 1811, if necessary and the twodimensional histograms are constructed 1812. The histograms may appearas shown in FIG. 10 and if so the region, which lies in the firstquadrant, corresponds to carbon wear.

[0092] The carbon wear can be calibrated 1814 by a curve of P specularlight versus carbon wear such as that shown in FIG. 14. FIG. 14 is atheoretical graph illustrating the change in P reflectivity and theabsolute P reflectivity verses the thickness of a carbon layer in nm.The theoretical curve shown in FIG. 14 has been computed from knowledgeof the complex indices of refraction of the layers of the thin film diskshown in FIG. 1 using a thin film analysis program called “Film Star”that is commercially available from FTG Software Associates, Princeton,N.J. Alternate ways to compute the curves of FIG. 14 are discussed byBorn and Wolf in “Principles of Optics” 6^(th) Edition CambridgeUniversity Press, 1997 beginning at page 51, and by Azzam and Bashara in“Ellipsometry and Polarized Light” North-Holland, 1987 pgs.270-315, forexample. The equations relating reflectivity of a thin film to theabsorbing layer thickness can be found in “Ellipsometry and PolarizedLight” referenced above on pages 283 through 288, for example. Theseequations or similar ones referenced in Born and Wolf may beincorporated into the computer software to automatically predict thefilm thickness from the reflectivity of the disk. In order to predictthe film thickness it is first necessary to know the complex indices ofrefraction of the carbon 104 and magnetic layers 106. The change inreflectivity of the points in the first quadrant of the histogram can berelated to the wear of the carbon through the theoretical change inreflectivity scale (on the right side of FIG. 14) such as the curveillustrated in FIG. 14. The absolute reflectivity scale on the left ofFIG. 14 can be used to compute the absolute thickness of the carbon. Thecomputation of the absolute thickness of the carbon requires knowledgeof the complex indices of refraction of the carbon and magnetic layers.A similar curve can be computed with the S polarized reflectance fromthe thin film disk.

[0093] The reflectance on the left and right scales of FIG. 14 are thosemeasured experimentally by the sum of P_(SP) and P_(SC). A system whichuses only the specular component of light will ignore the light whichhas been scattered and will give a measurement of carbon thickness orwear which is incorrect. An additional advantage of using the sum of thespecular and scattered components is that the signal from the carbonwear is essential doubled. This is because as the carbon wears the Pspecular component increases as well as the P scattered component. The Pscattered component increases since most of the P light penetrates theabsorbing film and scatters off the magnetic layer 106. As the carbon isthinned the amount of scattered light from the magnetic layer 106 willincrease, since there is less carbon to absorb the scattered light fromthe magnetic layer.

[0094] An alternative method for identifying the amount of carbon wearis to measure the k (complex part of the index of refraction) of thecarbon and use the percentage reflectivity change per angstrom of carbonwear. FIG. 15 is a graph illustrating the sensitivity of P polarizedlight reflectivity to carbon wear verses the k of carbon for a lightsignal having a wavelength of 650 nm and having an angle of incidence of58 degrees. FIG. 15 illustrates the relationship between the sensitivityof P polarized light to carbon wear versus the imaginary portion of theindex of refraction (k) of the carbon. The initial slope (at200-Angstrom thickness) of the curve in FIG. 14 is similar to theordinate illustrated in FIG. 15 where the changes in the ordinate aredue to the changes in the k of the carbon. The abscissa is the k of thecarbon and the ordinate is the sensitivity of the P specular light tochanges in carbon thickness, expressed in the percentage of P polarizedlight reflectivity change per Angstrom of carbon wear. Typical carbonshave k values near 0.4, making the sensitivity about 0.07 percent perAngstrom of carbon wear. This technique allows the detection of 0.01percent change in the reflectivity and, therefore changes in carbon wearof less than one Angstrom. The graph of FIG. 15 can be used to determinean approximate measure of the carbon wear. The initial slope of thecurve in FIG. 14 is given as the k=0.4 value illustrated in FIG. 15. Thefirst technique, using FIG. 14, has the advantage of accounting for thenonlinearity of the reflectivity change vs. wear. The second technique,using FIG. 15, has the advantage of simplicity. The analysis of thecarbon wear is aided by the use of a one-dimensional wear histogram. Thepixels in the image corresponding to carbon wear (the first quadrant ifthe angle of incidence is 58 degrees) are plotted in a histogram. Thisone-dimensional histogram has as the ordinate the number of pixels andthe abscissa is the amount of carbon wear as calibrated above. Thehistogram allows the user to display the amount of the surface which isworn (the number of pixels) versus the amount of wear. In this mannerthe user can select the amount of wear (a point on the abscissa) anddetermine the percentage of the surface area, which is worn above orbelow this amount of wear. This aids the comparison of different carbonsurfaces in their response to wear induced by the thin film head.

[0095] The analysis of the data is accomplished by analyzing the imagesof the thin film disk as a function of time. The disk is subject to someaction such as start/stops of the thin film head, which may alter thedisk surface as described above. Images of the disk are repeated atcertain time intervals and these images are analyzed in steps 702-716via the two dimensional histograms. An example of carbon wear analysisusing an in situ procedure is shown in FIG. 18. The steps of collectingimages and analyzing them 702-716 are repeated until the experimentcomes to a conclusion. The images are constructed by moving theapparatus 200 across a radius of the thin film disk with a very accuratestepper motor while rotating the disk at a high rate of speed (1000 to20000 rpm).

[0096] The spindle which rotates the disk at a high rate of speedcontains an encoder which produces 1024 pulses as it rotates through 360degrees, for example. This encoder is used to determine thecircumferential positions around the disk. The present inventionpreferably utilizes a very high-resolution determination of the positionaround the circumference of the disk. This is accomplished by using aphase locked loop to multiply the encoder signal by a selectable factorof up to 64 times. The phase locked loop, which multiplies the 1024encoder pulses, has the ability to track any velocity jitter in theencoder. This feature allows averaging of repeated revolutions to bedone with no loss of lateral resolution. That is, subsequent revolutionslie in phase with one another and when averaged, the resulting image isnot smeared by any jitter effect. Averaging is done to improvesignal-to-noise ratio.

[0097] The spectrum of spindle jitter is assumed to be related to thefrequency of spindle rotation, and limited to some multiple of it.Jitter can be due, for example, to the variations in torque from themotor poles. Regardless of spindle-frequency jitter, the encoder outputnonetheless exactly tracks data on the disk. It would be ideal thereforeto synchronize the data-acquisition clock to the encoder. In practice,there are limitations on the frequency at which the clock can be made totrack the encoder. In the phase-locked loop (which generates the clockfor the Analog to digital converter from the encoder) the encoder iscompared to the internal clock reference, generating phase-error pulses.The duty cycle of these pulses constitutes an error signal indicatingwhether the internal reference matches the encoder frequency or shouldbe adjusted to maintain tracking. To convert these pulses to an averagevoltage useful as an error signal requires a low-pass filter, whichlimits the tracking bandwidth. A conflicting parameter is theerror-signal ripple, which diminishes as the low-pass filter cutofffrequency is lowered. Ripple on the error signal leads to smallvariations in the frequency of the multiplied clock. This rise and fallof the clock frequency during each encoder period, while consistent fromscan to scan, and therefore not a threat to averaging could, if large inmagnitude, distort the appearance of data features. The repeatable partof the spindle jitter is not a threat to averaging (although it maydistort the image). The non-repeatable part of the jitter will smear outthe averaged image and this circuit will remove this smearing resultingin high resolution, high signal to noise images.

[0098] Since the encoder frequency is 1024 times the spindle frequency,compromise can be struck, placing the cutoff frequency of the low-passfilter at about 50-100 times higher than the spindle frequency, andabout 10-20 times lower than the encoder frequency. In this way, spindlejitter up to >50 times the spindle frequency is tracked, while the clockfrequency remains stable to within ± a few percent. Since the encoderfrequency varies widely, the cutoff frequency of the low-pass filtershould be adjusted to maintain the ratios set forth above. The 68:1encoder-frequency range of one embodiment of the present invention isdivided into seven 2:1 ranges, each of which uses a different, fixedfilter configuration set by switching the appropriate capacitor throughan analog multiplexer.

[0099] As described above, in the preferred embodiment, the measurementof the thin film disk properties is accomplished in situ. In analternate embodiment, the measurement of the thin film properties isaccomplished ex situ. FIG. 19 is a flow chart illustrating the methodfor measuring carbon wear using an ex situ procedure according to thepresent invention. One technique for measuring the thin film diskproperties ex situ is to test the thin film magnetic disk on a separatetest stand. This means that no reference image needs to be taken. Theuser places the disk on the spindle and the apparatus illustrated inFIG. 2 scans the disk for carbon wear. This scan provides a measure ofthe carbon wear, lubricant depletion, lubricant pooling, and surfaceroughness changes at the completion of the experiment. The user measures1902 P_(sp), P_(sc), S_(sp), and S_(sc). The images are high passfiltered 1904 to remove any background variations and the DC value ofthe reflectivity is removed 1906 by setting the average value of theimage to zero. The summed images P_(sp)+P_(sc) and S_(sp)+S_(sc) arethen computed 1908 and low pass filtered 1910 to remove noise, ifnecessary. The two dimensional histogram is then computed 1912 and thepoints corresponding to carbon wear are identified. The intensities ofthese points are related 1914 to the carbon wear in angstroms via suchcurves as FIG. 14 and FIG. 15.

[0100] The absolute thickness of the carbon may be computed by relatingthe sum of P_(sp) and P_(sc) or the sum of S_(sp) and S_(sc) via atheoretical model such as shown FIG. 14 to the carbon thickness. Thismethod is not limited to measuring carbon thickness but can be appliedto any reflective substrate which is coated with an absorbing coating.

[0101] Manufacturers make thin film disks with a known carbon overcoatthickness. The control of the carbon thickness is on the order of+/−10%. The knowledge of absolute thickness and the complex indices ofrefraction of the carbon allow one to construct calibration curves suchas FIGS. 14 and 15 and as a result one can determine the amount ofcarbon wear. The change in the thickness of the lubricant can bedetermined from the second or fourth quadrants of the two dimensionalhistogram. The calibration factors for the lubricant are taken from acurve, as described above. The calibration factor will depend at whatangle the particular embodiment is operating. If the embodiment isoperating at an angle between 53 and 71 degrees then the data falling inthe fourth quadrant corresponds to lubricant pooling and in the secondquadrant to lubricant depletion. The absolute thickness of the lubricantcan be determined by removing a section of the lubricant on the disk 100with a suitable solvent. The reflectivity corresponding to the step maybe measured in P or S specular reflectivity. This reflectivity changemay be related to the thickness of the lubricant via curves such as FIG.8. Curves such as FIG. 8 may be computed with software such as “FilmStar” that is commercially available from FTG Software Associates, inPrinceton, N.J. Either S or P reflectivity may be used but Sreflectivity is preferred since the sensitivity to lubricant in Sreflectivity is nearly independent of the k of the carbon when k is lessthan 1.

[0102] When the embodiment is operating at an angle between 53 and 71degrees the measurement of a step in the lubricant can be enhanced byperforming the ratio of the S image to the P image or vice versa. Thisgives an enhanced contrast to lubricant for two reasons: 1) the ratio ofP to S or S to P removes most reflectivity variations on the disk andshows only the step in the lubricant. 2) The response of S light to thelubricant step is opposite to that for P light and as a result the ratioimage increases the signal from the lubricant step. The sensitivity ofthe ratio of the S to P image (or P to S) to lubricant may be calibratedin a manner similar to the S or P specular images are calibrated forlubricant thickness.

[0103] Tribologists need to measure how lubricants move on the surfaceof thin film disks 100 since the motion of the lubricant is veryimportant in determining the durability of the carbon protective layer.The ratio may also be used to observe the motion (mobility) of thelubricant. The ratio gives enhanced contrast and removes reflectivityvariations unrelated to the lubricant step, as discussed above. Theratio also allows the user to remove the disk with a lubricant step andplace them under some environmental stress (such as humidity ortemperature) and then replace the disk and measure how far the lubricantstep has moved. This would not be possible without using the ratio, asthe absolute images do not have sufficient contrast to pick out thelubricant step.

[0104] The ratio of the P to the S images can also be used to identifydeep or unusual texture lines on the disk surface. This is possiblesince the ratio of these images is related to the index of refraction ofthe area being sampled by the optical beam. Unusual or deep texturelines have less carbon on them or contaminates within them. As a resultthe ratio image gives a strong contrast to deep or unusual texture linessince the lack of carbon or contaminates changes the index of refractionand as a result the ratio of P to S or S to P. The ratio of the P to theS images can also be used to identify contamination on the disk sincecontamination on the disk will cause the optical properties to change.In particular, the complex index of refraction will be changed by acontaminate beneath or upon the film and the ratio of P to S will showthis as a contrast between various areas of the disk. The individualimages will also show contaminates as changes in reflectivity, howeverthe ratio of P to S or S to P will show the changes more clearly sincethe ratio is constant except for areas where contamination is present.

[0105] In addition to measuring the lubricant properties and carbonwear, the present invention can simultaneously measure the surfaceroughness of the thin film disk 100 using the technique described above.The image of the roughness of the thin film magnetic disk gives thevariation of the roughness of the disk with position. The roughness orpolish of the disk is typically due to a mechanical polish, whichproduces polish marks, which are roughly circumferential in nature. Bymaking the Fast Fourier Transform (FFT) of the roughness image (obtainedfrom equation 1) or one of the specular images one can display thespatial frequencies of the roughly circumferential polish in the spatialfrequency domain. The FFT can be used to measure the angulardistribution of the polish lines, which is a parameter of interest inthe manufacture of thin film disks. The FFT of the roughness image givesthe angular distribution of the roughness of the texture lines and theroughness power density of the texture lines running along a particulardirection. FIG. 16 is an illustration of a FFT of a disk texture takenfrom the roughness image or one of the specular images as utilized bythe preferred embodiment of the present invention. A cut through thisFFT provides the roughness power density of the texture lines verses theangle and the angular width of the texture line roughness distribution.FIG. 17 is an illustration of a cut through the fast Fourier transformshowing the texture angles, width and texture power density distributionof a disk texture line pattern.

[0106] Another feature of the present invention is a method ofautomatically focusing the apparatus 200 shown in FIG. 2. Automaticfocusing can be accomplished by placing a laser zone textured disk onthe spinstand, which accompanies the instrument. A laser zone textureddisk is a magnetic thin film disk which has a series of laser meltedprotuberances placed in a spiral pattern near the inner diameter of thedisk. The laser-melted protuberances prevent the thin film head fromsticking to the disk when the disk is stopped. A spin stand is a teststand upon which the disk is placed which rotates the disk and simulatesthe action of a disk drive. The apparatus 200 shown in FIG. 2 is placedover the laser textured zone of the thin film magnetic disk and thespecular and scattered output is observed on an oscilloscope while thefocus is adjusted. When the instrument comes to a focus the specular andscattered signals from the laser bumps will reach a maximum value.

[0107] The properties of lubricants are sensitive to humidity, thereforeit is important to measure lubricant property as the humidity changes.Often the instrument 200 will operate in a high humidity environment.Heated optics allows operation of this embodiment of the presentinvention in high humidity environment. The optics of the instrument areheated to slightly above the environment temperature so that when usedin a humid environment water will not condense upon the optics. Onetechnique for heating the optics is to use the heat generated by theelectronics within the optical enclosure. An alternative technique is toplace a small heater in or near the optical assembly 200.

[0108] In an alternate embodiment of the present invention the aboveoptical surface analysis apparatus and method are used for rapidmeasurement of RMS roughness of laser bumps on thin film disk magneticmedia which can be correlated to laser bump heights. This is useful as aprocess control feedback in the manufacture of laser-bumps on thin filmdisks. The preferred embodiment of the present invention includes asmall spot size scatterometer (3-micron resolution) which allows one toresolve the scattered light from individual laser texture bumps. The RMSroughness of the individual laser bumps can be determined from the ratioof the scattered to the specularly reflected light as computed fromequation (1). The RMS roughness of individually resolved laser bumps isa function of the height of the laser bumps. Therefore the measurementof the RMS roughness of laser texture bumps can be used to monitor theheight and the height distribution of laser texture bumps. Thistechnique has the distinct advantage of being extremely fast (10 MHzdata rate)—orders of magnitude faster than conventional optical ormechanical techniques for determining laser bump height.

[0109] In an alternate embodiment of the present invention the aboveoptical surface analysis apparatus and method are used to help identifythe effects of burnish or glide head on the lubricant layer 102 and/orthe carbon layer 104. A burnish head is a low flying ceramic slider thatflies near the surface of the disk. In doing so it removes asperitesfrom the surface of the disk. A glide head is a low flying ceramicslider that is equipped with a piezoelectric sensor, in the preferredembodiment. The glide head is flown over the surface of the disk andwhen it encounters an asperity it sends a signal that indicates a defectis present on the disk.

[0110] It is typically difficult to observe the effects of a burnish orglide head on the lubricant layer 102 or the carbon layer 104 sincethere is no conventional system or method for observing these effects insitu, i.e., while in the process of burnishing or gliding the disk. Oneembodiment of the invention combines the system and method describedabove for measuring thin film disk properties with magnetics, friction,stiction, burnish heads and acoustic emission for glide in the mannerdescribed below. The combination permits inspection of the effects ofburnish and glide on the carbon layer 104, e.g., changes in roughness,texture, and/or carbon layer thickness.

[0111] During track following, which is when the thin film slider staysat one particular radius of the thin film disk for a prolonged period oftime, or during accessing on a thin film disk it is possible for theslider to deplete, pool, or degrade the lubricant layer 102. Thisembodiment of the present invention measures and analyzes these effectswhile they occur. For example, a layer of degraded lubricant can form onthe disk as a result of prolonged track following. This embodimentmeasures the lubricant layer 102 properties by measuring the effect thechanges in the lubricant has on the magnetic signals. The result ofchanges in carbon thickness are measured by changes in the amplitude ofthe magnetic signal. In addition, all of the measurements can be made insitu, i.e., without removing the disk from the spindle.

[0112] This embodiment of the present invention is a tool forcharacterizing surface properties such as roughness, lubricantdepletion/pooling, lubricant degradation, surface debris, and carbonwear. It measures and images the disk surface. There are tools with aspindle and a rotary actuator that can simulate the action of a diskdrive, they are commonly known as “spinstands”. It also simulates thewear and tear done by the magnetic head as it contacts the disk surfaceduring starting and stopping of the spindle. In addition, the spinstandcan include tools that measure other properties of the disk. Forexample, head to disk stiction/friction, magnetic signal amplitude ofthe disk, acoustic emission from contacts between the head and the disk,and sensors that detect and map surface features that protrude above themean fly-height of the head.

[0113] In one embodiment the above described capabilities are combined,which enables the data from each tool to be correlated to themeasurements of the other tools. This allows the user to correlate thedata because the various measurements can be done simultaneously orsequentially and in-situ. This significantly enhances the usefulness ofthe tool. For example, the mechanism behind the failure of a thin filmdisk during spindle start/stop can be better understood. In addition,the evolution of lubricant degradation/depletion, carbon wear, andsurface roughness changes can be followed as a function of the number ofspindle start/stops.

[0114] In combining this embodiment of the present invention with thespinstand, the optical component of the invention 200 and the spinstandrotary actuator and magnetic head are in very close vicinity. Theoptical components 200 optimally should not take up more than one halfof the disk area, otherwise it could collide with the spinstand rotaryactuator. The invention 200 has a miniaturized form; in particular theintegrating sphere is miniaturized because it is the closest to therotary actuator, which holds the magnetic head.

[0115] Another embodiment of the present invention combines theapparatus and method for measuring thin film properties, describedabove, with high resolution optical or mechanical tools such as anatomic force microscope (AFM) to quickly and easily identify damagedareas on a thin film disk. A conventional atomic force microscope, forexample model DI-5000 from Digital Instruments, Santa Barbara, Calif.,is capable of providing a very high resolution image of a surface buthas a very small detection range (viewing diameter), e.g., approximately100 micrometers. Accordingly, it is extremely difficult to find areas ona thin film disk that are damaged using conventional AFMs. However, theapparatus 200 and method described above easily and quickly locatesdamaged areas on a thin film disk 100. As described above, oneembodiment of the present invention identifies thin film disk damage inthe form of carbon wear, surface roughness, lubricant depletion,lubricant pooling, and lubricant degradation, for example. Thisembodiment of the present invention uses the optical analyzer apparatus200 to quickly and inexpensively locate damaged portions of a thin filmdisk 100. The apparatus 200 precisely identifies the damaged locations.The AFM is directed to examine the precisely identified damagedlocations. This embodiment enables the AFM to locate and perform a verydetailed analysis of the damaged portion of the disk. Accordingly, thisembodiment of the present invention enables a user to quickly andinexpensively locate one or more damaged portions on a thin film disk100 and enables the user to direct an AFM directly onto the damagedportions much more quickly than is possible using conventional systemsand methods.

[0116] A specific example is the study of carbon wear on laser bumps, itis desirable to be able to find specific laser bumps (from hundreds ofthousands of laser bumps) which have carbon wear. It is preferable tolocate these worn bumps quickly and to study these laser bumps under ahigh power and high resolution measurement or imaging tool. The presentinvention is able to locate the laser bumps quickly but it does not havethe resolution for studying the laser bumps in extreme detail.Conversely, high resolution tools, such as an Atomic Force Microscope(AFM) are too slow to analyze large number of bumps. A combination ofthe two types of tools provides the advantages of both tools. Laserbumps of interest (with carbon wear) can be found quickly using thepresent invention, the position encoder of the spindle holding the diskcan track their locations. The same spindle can also calibrate therelative location of the optical beam position to the location of theAFM. These laser bumps can then be placed under the high-resolution tool(e.g. AFM) for further study.

[0117] Another embodiment of the present invention is a system andmethod for performing high temperature film measurement. FIG. 20 is anillustration of a high temperature thin film measurement system 2000according to one embodiment of the present invention. The hightemperature system 2000 is a reverse angle illustration when compared tothe view illustrated in FIG. 2. The system 2000 is capable of measuringthe carbon film thickness and wear, lubricant film thickness andthickness variation, surface roughness, and degradation of lubricant.The system design is similar to design set forth above with respect toFIG. 2, for example. One variation is that the high temperature thinfilm measurement system 2000 allows operation at high temperatures,e.g., 80 degrees Celsius. The high temperature thin film measurementsystem 2000 uses a zero order temperature compensated quartz half waveplate 2004 available from, for example, CVI Laser, Albuquerque, N.M. anda high temperature laser diode 2002 available from, for example RohmCo., LTD. Kyoto, Japan. The zero order quartz half-wave plate 2004 ismounted in a rotatable housing that can be rotated through 45 degrees bya miniature motor, for example Maxon Precision Motors, Burlingame,Calif. model No. 118426 using gears 2008 that are commercially availablefrom W. M. Berg, East Rockaway, N.Y. Rotating the half wave platethrough 45 degrees will rotate the polarization by 90 degrees. The hightemperature thin film measurement system 2000 also includes anintegrating sphere 218 and baffle 226 that can be similar to thosedescribed above with reference to FIG. 2. The integrating sphere 218 iscut out of the interior of a cubic aluminum block. The high temperaturethin film measurement system 2000 also includes a focusing lens 216 anda collimating lens 220 similar to FIG. 2. The high temperature thin filmmeasurement system 2000 also includes feedback circuitry substantiallyidentical to that described in FIG. 4 for controlling laser intensity,and amplifying, signal conditioning and digitizing electronicssubstantially identical to that described in FIG. 6.

[0118] One problem to be solved is to develop a high temperature filmmeasurement system. Hard disk drive and disk manufacturers need to testthe carbon and lubricant on their disks at relatively high temperatures,e.g., 80 degrees Celsius. This can be accomplished by making a filmmeasurement system which can be operated at these temperatures. Themeans to accomplish this is to use a mechanically rotatable temperaturecompensated zero order half wave plate 2004 together with a hightemperature laser diode 2002. The laser which is used is available fromRohm Co., LTD. In Kyoto, Japan and the model number is RLD-78MAT1. Thisis a 780 nm laser diode which has low noise and can operate continuouslyat 80 degrees C. The temperature compensated zero order half wave plateis available from CVI Laser Corp. in Albuquerque, N.M., USA. All theother optical and electrical components are rated at temperatures higherthan 80 degrees C., so the resulting system can be operated up to 80degrees C. and it may be operated at a humidity of 80% RH (relativehumidity).

[0119] Disk drive companies and their suppliers test the thin film disksand the completed disk drives in environments of up to 70 degrees C. ata relative humidity of 80%, for example. Conventional systems use alaser which is either Peltier cooled or an uncooled intensity stabilizedlaser. The Peltier cooled laser suffers from several problems, namely,when attempting to use a cooled laser (cooled for example to 25 degreesC.) in a chamber at a temperature of 70 degrees C. and a relativehumidity of 80%, water will condense on the cooled surface of the laserthus damaging the optical surface. Attempts to operate the Peltiercooler at a temperature of greater than 50 or 60 degrees C. can damage aconventional laser. The embodiment shown in FIG. 20 uses an uncooledlaser 2002 that has been developed for continuous operation at atemperature of 80 degrees C. The system described in FIG. 20 will alsowork at a relative humidity of 80% since the laser actually operates ata temperature slightly greater than its ambient surroundings.

[0120] Another problem which needs to be overcome in high temperaturesystems is a means to switch the polarization between the P and Spolarizations. Conventional liquid crystal variable half wave plates 206cannot be operated above 50 degrees C. A solution to this problem is touse a zero order thermally compensated half-wave plate. The type of waveplate has a thermal compensation which allows it to operate totemperatures greater than 80 degrees C. The zero-order half wave plateis mechanically rotated in order to switch the polarization between theP and S states. This is accomplished by a miniature electric motor 2006and gears 2008. In order to reduce the effect of temperature on theelectronics the preamplifier and laser regulator board 214 are locatedoutside the high temperature environment. The connections between thephotodetectors and the board 214 are made with cables.

[0121] The system shown in FIG. 20 operates in a manner similar to thatshown in FIG. 2. This system has a input aperture 2016 in theintegrating sphere 218 which is slightly larger than the optical beam sothat the beam is not eclipsed by the opening. The integrating sphere 218has a hole 2022 in its bottom that allows the beam to strike the diskand reflect out of the integrating sphere through an aperture 2024.Aperture 2024 is slightly larger than the beam to allow the beam to passthrough without being eclipsed. The location of aperture 2024 is lessthan 1 cm from the surface of the disk. The diameter of aperture 2024can control the minimum spatial frequency of roughness, which the devicecan measure according to equation (3), discussed supra. The integratingsphere includes an opening at its top 2018 to allow scattered light tostrike the scattered photodetector 224B. The specular beam is directedonto a collimating lens 220 which prevents the beam from spreading.After passing through the collimating lens it passes into a miniatureintegrating sphere 2028 through an opening 2030. The integrating spherereduces the sensitivity of the photodetector to disk distortion andrunout. A distorted disk is one which differs from a perfect flat plane.The manufacturing process or the process of clamping the disk on thespindle can cause distortion of a disk. Disk runout is the motion of thedisk in the vertical direction caused by imperfection in the spindle andmechanical vibrations of the disk. The specular intensity is detectedvia a hole 2032 in the miniature integrating sphere with a specularphotodetector 224A. The hole 2030 is designed to be larger than thecollimated specular beam so that the beam is not eclipsed by the hole.The integrating sphere 2028 is rotated slightly in the plane of thepaper so that its entrance port is not perpendicular to the beam. Thismeans that the reflected signal from the back of the integrating sphere2028 will not retro-reflect down the optical path into the scatteredlight integrating sphere 218. Retro-reflect means to reflectsubstantially directly down the path of the incoming laser beam. Theamount of reflected light which gets into the integrating sphere 218 isfurther reduced by using an opaque black baffle 2026 placed between theintegrating sphere 218 and the collimating lens 220. Another means ofreducing the sensitivity of the specular photodetector 224A to diskdistortion is to place a diffuser (not shown) such as the diffuser 222shown in FIG. 2 in front of the specular photodetector 224A.

[0122] An optical surface analyzer, for example the apparatus 200, 2000described above, measures thickness changes induced by wear of multiplelayers of thin film coating on a reflective surface, e.g. magnetic diskmedia. Another feature of the present invention is a system and methodfor separating and identifying the signals from the different layers ofthe thin film using a 2-D histogram. This method is described in detailby Steven Meeks, W. Weresin, and H. Rosen in ASME Journal of Tribology,vol. 117, pg. 112, published in January 1995 which was incorporated byreference above.

[0123] The 2-D histogram is generated by the instrument software andallows the user to select specific areas of interest in the histogram,such as the carbon wear signal, by manually tracing a line around thatarea. The software then finds and highlights the location of theselected area of the histogram on the image of the disk by tracing backto the image source location. To construct a two dimensional histogramsmall regions (known as buckets) are defined in the P, S plane (thespace of the histogram) which have a certain ΔP by ΔS dimension. Eachcoordinate pair (x,y) in the real space image is selected and itscorresponding bucket into which its P, S coordinate falls is identified.After going through the entire image the total number of points in eachbucket is identified and a color, for example, is assigned based uponthe number of points in the bucket. The completed two dimensional imageis known as a two-dimensional concentration histogram. Thetwo-dimensional concentration histogram separates the regions oflubricant pooling, depletion, carbon wear and debris into separateregions. Debris are products generated as a result of the wear processon the disk. In addition, the slope of the histogram is related to theindex of refraction of the layer being altered. One technique forgenerating a two-dimensional histograms is also discussed in detail byBright and Marinenko, in Microscopy: The Key Research Tool, Vol. 22 pg.21, 1992 in an article entitled “Concentration Histogram Imaging: AQuantitative View of Related Images”.

[0124] Three embodiments of the present invention each use a method forautomatically selecting specific areas of interest, such as the carbonwear signal, degraded lube, lubricant pooling/depletion, debris anddefects without operator intervention and then using this information tooptimize the type of carbon or lubricant or to analyze the failure of adisk drive.

[0125] The three methods are: (1) performing a symmetry operation abouta centroid, (2) subtracting a reference histogram and (3) performing anand/not operation with a reference histogram.

[0126] The three embodiments can be performed in a conventional computersystem, e.g., a personal computer, a microcontroller, a single chipcomputer, a network. As an example, these embodiments of presentinvention can be performed on a conventional computer system, e.g., aconventional personal computer, or a microcontroller for example, suchas that illustrated in FIG. 21. The computer system illustrated in FIG.21 includes a conventional processor, such as a Pentium II 400 MHzprocessor, a conventional storage unit 2104, a conventional I/O unit2106 and conventional memory 2108. In one embodiment of the presentinvention the memory 2108 can include software related to the operatingsystem 2110, e.g., Windows 98 that is commercially available fromMicrosoft Corporation, Redmond, Wash. In addition, some embodiments ofthe present invention can include one or more of the symmetry unit 2112,the histogram subtraction unit 2114, and the AND/NOT unit 2116. A moredetailed description of the symmetry unit 2112, the histogramsubtraction unit 2114, and the AND/NOT unit 2116 is set forth below.

[0127] One embodiment of the present invention is to have the symmetryunit 2112 perform a symmetry operation about the centroid of thehistogram to create a symmetric histogram about the centroid. FIG. 22 isa flowchart illustrating the operation of the symmetry unit 2112according to one embodiment of the present invention. The symmetry unit2112 receives 2202 the histogram. The two-dimensional histogram can becreated in the manner described above. The symmetry unit 2112 determines2204 the centroid of the histogram. One technique for identifying thecentroid of a 2D Histogram is now set forth. The symmetry unit firstconverts the 2D histogram into a binary representation, e.g., anynon-zero pixel values become “1” otherwise the pixel takes the value of“0”. Second, from this binary image, a skeletonization/Medial Axistransformation in performed. Skeletonization is a process for reducing abinary image in to a skeletal remnant that largely preserves the extentand connectivity of the original region. Skeletonization is one of themorphological filters for Digital Image Processing. Further processing(e.g., pruning by thinning or erosion) may be necessary to produce askeleton that is free of spurious spurs which can be introduced duringthe process of skeletonization. Next, other morphological filters can beused in addition to or separately from ‘skeletonization’ to get betterrepresentation of the centriod, such filters include: thinning, which isessentially reducing a binary image shape into a single pixel thickness,and erosion, which is a process to erode away the boundaries of theoriginal region. This operation can remove speckle noise on the 2Dhistogram image, for example. One example of the skeletonization processis described in detail in ‘Digital Imaging’ by Howard E. Burdick,McGraw-Hill, 1997 which is incorporated by reference herein in itsentirety.

[0128] The symmetry unit 2112 mirrors 2206 the left side of thehistogram about the centroid line onto the right side and subtracts 2208the mirrored left side from the right side of the histogram. Theresulting histogram represents the asymmetric portions of the histogramwhich can be identified via a look up table as containing carbon wear,debris, defects, etc. Once the areas of interest are identified,parameters such as percent of surface area covered by wear, depth ofwear, degraded lube, etc., can be calculated automatically.

[0129]FIG. 25 is an example of a simplified two-dimensional histogramimage according to an embodiment of the present invention. The signalfrom the thin-film thickness change (e.g. carbon wear) is shown as theshaded area 2502. The goal is to remove the non-shaded portion of thehistogram from the data. This area can be automatically isolated fromthe rest of the image by calculating the centroid line 2504 that is aclose approximation of the line of symmetry of the non-shaded area. Theregion 2506 of the histogram which is symmetric about the centroid line2504 of the histogram image can then be removed from the data. In thisexample, the data to the left of the centroid line 2504 and itssymmetric part on the opposite side of the centroid line 2504 is to besubtracted 2208 from the histogram. What remains is the shaded area 2502which is the signal from the worn area. This area corresponding tocarbon wear can be projected on to the P_(sum) axis and with propercalibration they can give a quantitative amount of the carbon wear. Thesame calibration factors discussed earlier in this text may be used tocompute the amount of carbon wear. This technique lends itself to theautomatic analysis of data. For example, the automatic analysis of thepercent of the surface worn and a 1D histogram of the wear of thecarbon.

[0130] The 2D histogram consists of values at (x, y) bin positions whoseamplitude is given by counting the number of points from two spatiallyidentical images of amplitudes x and y which fall within the binpositions. Bin positions are the locations of small areas of dimensionsΔx by Δy whose purpose is to serve as buckets which can accumulate andcount the number of points which fall into these dimensions at aspecified location. The 2D histogram may be formed by any two imagesmeasured over the same spatial location.

[0131] The analysis of the portions of the histogram which remain afterthe subtraction of the symmetric part of the histogram can be analyzedbased upon the values of the P_(sum) and S_(sum). FIG. 26 is a chartillustrating an analysis technique according to one embodiment of thepresent invention.

[0132] If the Psum/Ssum are both positive, i.e., the values fall intoquadrant 1, the tested area of the disk has only carbon wear. If thevalues fall into quadrant 2 then the tested area of the disk haslubricant depletion. If the values fall into quadrant 3 then the testedarea of the disk has degraded lubricant and/or debris, or unusualtexture lines. If the values fall into quadrant 4 the tested area of thedisk has mixed carbon wear and degraded lubricant or lubricant poolingor organic pooling.

[0133] Another embodiment of the present invention uses a referencehistogram which is subtracted from the histogram of the disk to bemeasured. FIG. 23 is a flow chart illustrating the operation of thehistogram subtraction unit 2114 according to one embodiment of thepresent invention. The histogram subtraction unit 2114 receives 2304 afirst (reference) histogram that can represent measurements from a thinfilm disk which is very similar to the disk to be measured but has nodamage on its surface. For example, the first histogram can be thehistogram of the disk before testing, or a disk from the same batch (asister disk) or the histogram from the untested side of the disk to bemeasured, for example.

[0134] If a reference histogram is not available from a sister disk, areference histogram can also be obtained in one of several ways. Thefirst way is to construct a 2D histogram from a subset of the currentimage (the surface under test). The subset is chosen on a region of thedisk under test which has no damage, so that it provides a histogram ofa virgin surface. Alternatively, a representation of the background (thereference histogram) can be obtained by performing a ‘traceforward’operation on an undamaged region of the image that makes up the 2DHistogram of the disk under test. The ‘traceforward’ operation isperformed on a subset of the image that is deemed to be free of defectsor damage. The ‘traceforward’ operation is described in detail by Brightand Marinenko, in Microscopy: The Key Research Tool, Vol. 22 pg. 21,1992 in an article entitled “Concentration Histogram Imaging: AQuantitative View of Related Images”. The resulting collection of pixelswhich is now in the 2d histogram domain can be used as representativepixels of a background histogram for that particular image.

[0135] One technique for calculating the traceforward is to (1) firstobtain the region on the original image where the traceforward isdesired, and (2) for each pixel inside the region selected for thetraceforward on the original images that form the 2D histogram alocation in the 2D histogram can be calculated from the pixel value ofthe first image source and the second image source. The pixel valueswill fall into a particular x-axis bin and y-axis bin respectively, onthe 2D histogram.

[0136] The histogram subtraction unit 2114 also receives 2304 a secondhistogram representing the disk after testing, for example. Thehistogram subtraction unit 2114 determines 2306 whether all points havebeen selected from each histogram. If not, the histogram subtractionunit 2114 selects 2308 a point in each histogram, e.g., A[x,y] andB[x,y]. The histogram subtraction unit 2114 then generates a resultinghistogram (R) by subtracting 2310 the point from the histogram undertest, B[x,y], from the reference histogram point, A[x,y], (or vice versawith the appropriate modification to the analysis using FIG. 26). Theresulting histogram is separated into the four separate quadrants asdescribed above with reference to FIG. 26.

[0137] Another embodiment of the present invention uses a referencehistogram which a NOT operation is performed between a referencehistogram and a data histogram followed by an AND operation. FIG. 24 isa flow chart illustrating the operation of the AND/NOT unit 2116according to one embodiment of the present invention. The AND/NOT unit2116 receives 2402 two histograms, e.g., a reference histogram (B) and adata or test histogram (A), as described above. The AND/NOT unit 2116selects 2404 a point (x,y) in each histogram. If the value of theselected point in the reference histogram is greater than zero then theAND/NOT unit 2116 sets 2408 the corresponding point in the resultinghistogram equal to zero. If the value of the selected point in thereference histogram is not greater than zero then the AND/NOT unit 2116sets the point in the resulting histogram equal to the selected point inthe test histogram, i.e., R[x,y]=A[x,y].

[0138] The AND/NOT unit 2116 uses a reference histogram in which an ANDoperation is performed between the reference histogram and the datahistogram followed by a NOT operation. This yields a resultant histogramwhich contains only those regions which are not common to both the dataand reference histograms.

[0139] The resulting and/not histogram is then segmented as shown aboveand each quadrant is labeled with the above interpretation. FIG. 27illustrates one example of histogram analysis according to the AND/NOTunit of the present invention. The data from each histogram quadrant canbe traced back to the original data image. The amount of surface areacovered in the original images by each of the regions shown in FIG. 27can be computed and displayed. For example, the total amount of thesurface which has carbon wear can be computed from the traceback. Thedepth of the carbon wear can be computed by calibrating the amount ofcarbon wear corresponding to the reflectivity change.

[0140] The above described process can be applied to theP_(sum)=P_(scattered)+P_(specular) Vs S_(sum)=S_(scattered)+S_(specular)histogram or any of the components: P_(scattered), P_(specular),S_(scattered), S_(specular) in any combination versus any othercombination. The reference histogram may be computed from an untestedsister disk, the opposite side of the tested disk, a sub-image histogramcomputed from an undamaged area of the disk under test or from atraceforward computed on an undamaged area of the disk under test, asdiscussed above.

[0141] For example, FIG. 28 illustrates one example of histogramanalysis using P specular or S specular versus P scattered or Sscattered variable according to the AND/NOT unit of the presentinvention. In this case the finger 2802 extending into the secondquadrant corresponds to corrosion products on the disk surface. Theslope of the centroid 2804 of this finger 2802 is related to the indexof refraction of the corrosion products. Since different materials havea different index of refraction and hence a different slope, this allowsthe user to separate debris, corrosion products and texture lines. Thesecond area 2806 along the horizontal axis of FIG. 28 is the regionwhich has been removed via the AND/NOT operation. The amount of thesurface covered by debris or texture lines is easily computed bycalculating the number of pixels contained in the finger area 2802 ofquadrant 2. The finger 2808 corresponds to debris on the disk. The slopeof the centroid 2810 of this finger 2808 is related to the index ofrefraction of the debris on the disk surface. This allows the user toquantitatively separate and identify (from the slope of the centroid)the different particles and corrosion products on the disk. Asubstantially identically analysis may be computed from a 2-dimensionalhistogram of the P specular versus the S specular light. In this casethe two fingers 2802 and 2808 will be extending into the third quadrant.The two fingers will still be separated by a different slope in a mannersubstantially identical to FIG. 28.

[0142] The above discussion has focused on a means to measure themagnitude of the S and P polarized beams and their appropriate scatteredcomponents. A means for improving the sensitivity of existing technologyis to add the ability to measure the phase between the S and Pcomponents of the wave. The measurement of the phase shift between the Sand P components will give a 4 to 20 times improvement in thesensitivity to the measurement of thin films and defects. This can beaccomplished by illuminating a thin film disk with linearly orelliptically polarized light that contains both P and S components oflight, for example. The detected light can then be analyzed to measurethe phase shift between the P and S components. Simultaneously you canalso measure the amplitude of the P and S components, the scatteredcomponents, and the Magneto-Optic Kerr rotation. If a bi-cell orposition sensitive detector is added to the optical design then it ispossible to also measure the displacement (height or depth) of featureson the surface. Accordingly, one embodiment of the present inventionsimultaneously performs film thickness measurements, surface roughnessmeasurement, reflectivity measurement, magnetic imaging, and opticalprofiling. The preferred embodiment of this instrument is shown in FIG.29.

[0143]FIG. 29 is an illustration of a combined reflectometer,scatterometer, phase shift microscope, magneto-optic Kerr effectmicroscope and optical profilometer 2900 according to one embodiment ofthe present invention. This invention uses a multi-mode,multi-wavelength laser diode 2902 which is available from Rohm Co., LTDKyoto, Japan as model number RLD-78MV and a polarizer 204 which isadjusted for P polarization and improves the extinction ratio of thelaser. The next element is a mechanically rotatable half wave plate 2904that is available from CVI Laser Corp. which can be used to rotate thepolarization between 45 degrees, and P or S polarization's. Alternativetechniques for rotating the polarization is to rotate the laser diode2902 or to use a liquid crystal polarization rotator such as modelLPR-100 available from Meadowlark Optics, Frederick, Colo. The latterembodiment has the advantage of being a purely electronic means ofpolarization rotation and as a result there is no possibility of beammovement when the polarization is rotated.

[0144] The non-polarizing beam splitter 208 is used to provide areference beam to stabilize the laser diode intensity. The focusing lens216 creates a small spot on the surface of a thin film disk 100. Theintegrating sphere 218 is used to measure the scattered light for thepurposes of computing the surface roughness and measuring debris orcorrosion on the disk surface. The holes in the input 2920 and output2922 are designed to pass the specular optical beam and collect thescattered light. The low spatial frequency limit of the scattered lightis determined by the aperture diameter of the exit hole 2922. Thescattered light is measured with a photodetector 224B placed at an exithole 2926 at the top of the integrating sphere. The integrating sphere218 has a hole 2924 in the surface nearest the disk to allow the beam toreflect from the surface of the thin film disk 100.

[0145] After reflecting from the disk, the beam passes through anon-polarizing beam splitter 208 and approximately one-half of the beampasses into a bi-cell detector 2916 available from UDT Sensors, Inc.,Hawthorne, Calif. In one embodiment of the present invention the bi-celldetector includes two identical detectors which are separated by a smalldistance (typically less than 25 microns of separation). The output ofthe bi-cell passes into a differential amplifier 2918 that subtracts thetwo outputs. The subtracted output is digitally normalized by the sum ofthe outputs of 2912 and 2910 to remove any laser intensity or diskreflectivity changes. The normalized output is proportional to the slopeof the surface since differential detector 2916 measures the movement ofthe beam due to surface slope changes. The resulting image is a twodimensional map of the slope of the surface. The displacement of thesurface (height or depth) can be obtained by integrating the slopeversus position signal. The signal from the bi-cell 2916 can also beused to provide an autofocus signal for the optical signal. The opticalassembly is calibrated and focused when it is first assembled. Thereading of the voltage from the differential amplifier 2918 can berecorded in a storage device, e.g., the PC memory. When a differentthickness disk 100 is placed beneath the optics a small motor (notshown) can move the head up or down until the differential amplifier2918 reading is the same as that originally recorded when the opticalassembly was first assembled. When this is done the distance of theoptical assembly from the disk 100 remains constant and the opticalassembly remains in focus.

[0146] The bi-cell as described above will measure the circumferentialprofile of the surface. A quad-cell, which is also available from UDTSensors, Inc. Hawthorne, Calif., may be used to measure both thecircumferential and radial profiles of the surface. The quad-cell is afour-element detector with four identical elements as shown in FIG. 34.The two elements (1 and 2 in FIG. 34) which are oriented in thecircumferential direction may be summed together to form one half of thebi-cell detector 2916 and the other two elements (3 and 4) may be summedto form the other half of the bi-cell 2916. This configuration will givethe circumferential profile as described above. If elements 1 and 3 aresummed and 2 and 4 are summed then these two halves may be used to givethe radial profile. In this manner both the radial and circumferentialprofiles may be obtained from a single scan of the surface.

[0147] After passing through the non-polarizing beam splitter the otherhalf of the beam is recollimated with a lens 220. It then passes througha mechanically rotatable quarter wave plate 2906 available from CVILaser Corp. The beam is then polarization split with a Wollaston prism2908 available from CVI Laser Corp., for example, and each polarizationcomponent is detected with a separate photodetector. The plane of theWollaston prism (the plane of the S and P components) is adjusted at 45degrees to the plane of incidence. The first mixed component of the beam(which includes both P and S components with respect to the plane ofincidence) is directed to a conventional photodiode 2912 available fromHamamatsu Corp., for example, and the second mixed component (whichincludes both P and S components with respect to the plane of incidence)is directed to a conventional photodiode 2910. The photodiodes have adiffuser 210 placed in front of them to reduce the residual positionsensitivity of the photodiodes. The difference between thephotodetectors is proportional to the cosine of the phase differencebetween the first and second mixed components coming from the Wollastonprism. As a result this instrument can get different types ofinformation when used in different modes.

[0148] When the polarization is adjusted to P, the P specular and Pscattered light is measured resulting in sensitive measurements ofcarbon thickness and carbon wear. The P specular signal is obtained byrotating the half wave plate 2904 so that the incident polarization isP. The P specular signal is given by the sum of the signal from 2912 and2910. When the polarization is adjusted to 45 degrees (exactly between Pand S polarization) the instrument is most sensitive to measurements ofthe phase change induced by changes in the thickness of the thin filmson the disk surface. In the phase shift mode the instrument measureslubricant thickness and carbon thickness on thin film disks. The phaseshift is measured by taking the difference between the signals measuredat 2912 and 2910. This gives an output that is proportional to thecosine of the phase difference between the first and second mixedcomponents of the wave. The orientation of the quarter wave plate 2906is adjusted to optimize the sensitivity to lubricant and carbon wear orthickness. The individual components may also be measured; that is, thefirst and second mixed components of the 45 degrees polarized light.These are measured simultaneously with the phase shift and the scatteredlight.

[0149] The first and second mixed components of the 45 degree linearlypolarized light are referred to as S_(Q) and P_(Q). When thesecomponents of the phase shift are plotted in a two dimensionalconcentration histogram the interpretation of the data becomes as shownin FIG. 31. Carbon wear is seen in the second quadrant, carbon thicknessvariation on the disk surface is the length of the body of thehistogram, debris is in the third quadrant, and degraded lube and lubepooling is in the fourth quadrant.

[0150] When the polarization is adjusted to S polarization theinstrument will be able to measure the S specular and the S scatteredlight and, as a result, obtain the surface roughness and otherproperties of the sample. The S specular signal is given by the sum ofthe signal from 2912 and 2910. The angle of incidence shown in FIG. 29is 58 degrees but angles greater or less than 58 degrees will work aswell. The longitudinal Kerr effect can be measured by operating theinstrument in any of the linear polarization's, i.e., P, S or 45degrees. Rotating the quarter wave plate 2906 to achieve maximumsensitivity to the magnetic pattern optimizes the Kerr effect signal.The orientation of the quarter wave plate which optimizes the Kerreffect may be different from that which optimizes for lubricant andcarbon sensitivity. As a result the quarter wave plate is made to beremovable, for example, so that two different and separately optimizedplates can be used for the different applications. A differentembodiment would have a miniature motor to rotate the orientation of thequarter wave plate so as to optimize the signal for the Kerr effect orlubricant and carbon mode. The different polarization's will require adifferent quarter wave adjustment to achieve optimization. When in thismode the instrument finctions as a Kerr effect microscope. In thepreferred embodiment the S polarization is used to image thelongitudinal Kerr effect.

[0151] When a defect is observed on a disk with the instrument describedin FIG. 29 then it is desirable to be able to mark the position of thedefect with a mechanical scribe. Such a scribe is shown in FIG. 30. Theintegrating sphere 218 and related components have been removed from thedrawing in order to clearly show the scribe. In the actual instrumentboth the scribe and the integrating sphere are present. The scribeconsists of a supporting apparatus 3002, a linear slide 3004, a diamondscribe 3006 and an air cylinder actuator 3008. The air cylinder actuatorholds the diamond scribe away from the surface until it receives acommand from a PC to release the scribe. The scribe is attached to thesame optical body that holds the optical components of FIG. 29. Thismeans that the scribe can use the same linear stage as the optical bodyduring the scribing process. A process of marking a disk with a scribeis as follows: (1) locate the defect with the instrument described inFIG. 29; (2) from the image displayed on the PC video monitor indicateto the software that this is the defect to be marked (scribed); (3) thesoftware then rotates the disk 100, which is attached to a spindle withan encoder, so that the defect is at the same angle as the scribe; (4)the scribe is moved to the radius of the defect with the stage whichmoves the optical head and when in the close vicinity of the defect theair cylinder actuator is activated and the scribe drops onto thesurface; (5) the stage is moved a specified distance with the scribe incontact with the disk, this leaves a mark in the vicinity of the defect.This mark can be used to find the defect for subsequent analysis.Multiple marks can be placed in the vicinity of the defect if desired.

[0152] The instrument described in FIGS. 29 and 30 can simultaneouslymeasure the optical profile (height and depth) of the surface, the S andP components of the reflectivity, the phase shift between the P and Swaves and the scattered light. It is also capable of measuring theMagneto-optic Kerr effect of magnetic films and it has the ability toscribe defects for later analysis.

[0153] The measurement of the phase shift between the S and P componentsof the optical wave requires a means to stabilize the long-term phasedrift of the diode laser. This can be accomplished by the use of areference mirror. The reference mirror is a stable optical surface suchas a gold mirror or a section of a thin film disk. The reference mirroris calibrated when the instrument is first set up by measuring andrecording the phase shift of the reference mirror. At times after theinitial calibration of the instrument the reference mirror is measuredprior to a measurement of the sample. Any deviation of the referencemirror reading from the initial reading is recorded and subtracted fromthe measurement of the sample readings. This insures that the phaseshift reading from the surface under measurement (the thin film disk100) will remain stable over time. The same procedure can also beapplied to the measurement of the S specular and P specular signals. Inthis case the when the instrument is calibrated the values of the Pspecular and S specular signals measured on the reference mirror arerecorded and deviations from these values are used to correct thespecular data. This removes any drift from the P and S specular signals.

[0154] The above discussion is relating to an instrument, which has anangle of incidence that is near 60 degrees from the vertical. Similarideas can be applied to a machine operating at angles less than orgreater than 60 degrees. When the angle of incidence changes theinterpretation of the various quadrants of the histogram will changeaccording to the discussion given earlier.

[0155] As describe above, not only can this invention measure thin filmson magnetic disks but it can also measure the same parameters on siliconsemiconductor wafers. More particularly, the present invention canmeasure the film thickness, surface profile, surface scatter fromdefects, areas contaminated with unwanted thin films or particles andreflectivity changes due to missing or damaged films. One example of asemiconductor application is the measurement of the properties of thechemical mechanical polishing process (CMP). The CMP process is used toplanarize the surface of a coated and patterned silicon wafer. Tocontrol the CMP process engineers want to measure the amount of materialremoved from the metal region as compared to the oxide region. Since theoxide and the metal areas have different mechanical properties they willpolish at different rates. This results in what is commonly known as“dishing” and “erosion”. Erosion is the average amount of displacementof the polished metal lines below the surrounding oxide surface as aresult of CMP polishing. The narrow metal lines are interspersed withoxide and the amount of displacement of the metal lines below theirimmediate surrounding oxide is known as dishing. An illustration of thisis shown in FIG. 32. CMP process engineers wish to control the amount ofdishing and erosion. FIG. 33 illustrates the measurement of dishing anderosion by the measurement of the optical phase shift versus position ona die on a patterned silicon wafer. The erosion shown on FIG. 33 isgreatest nearest the edge of the metal, that is at positions 93 and2117. At these same positions the dishing is a minimum. The opticalphase shift directly measures the thickness changes of the oxide.Therefore in the phase shift mode the assumption is that the layerbeneath the oxide is flat. The same dishing and erosion can be measuredwith the optical profilometer 2900, described above. The opticalprofilometer 2900 directly measures the slope changes on the surface dueto erosion and dishing. The slope may be integrated to give the actuallyoptical profile. The advantage of the optical profiler is that it givesthe height changes independent of any thickness changes of theunderlying films.

[0156] While conventional systems have used a mechanical profiler tomeasure the same parameters as shown in FIG. 33 some of the advantagesof the present invention are that it can measure the entire siliconwafer in approximately one minute and is capable of generating a threedimensional image as opposed to merely generating a single line scan asis generated by conventional mechanical profilers. The three-dimensionalimage of the entire wafer gives the user process uniformity information.That is, the user can rapidly determine how the polishing process isvarying across the diameter of the wafer. This valuable information isvery difficult and time consuming to generate using conventionalmechanical profilers.

[0157] Another benefit of the present invention is that the instrumenthas a small footprint that allows it to be integrated within an existingsilicon process or metrology machine. For example, it can be placedwithin a CMP machine in place of the wafer flat or notch finder module.This means that this invention will use no more clean room floor spacethan current process or metrology machines.

[0158] While the invention has been particularly shown and describedwith reference to a preferred embodiment and several alternateembodiments, it will be understood by persons skilled in the relevantart that various changes in form and details can be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method for measuring at least one of a Kerreffect and lubricant thickness on a first object, comprising the stepsof: transmitting a first light signal toward the first object; receivinga reflected light signal that has reflected off said first object, saidreflected light signal comprising a first mixed reflected polarizedcomponent having a first phase and a second mixed reflected polarizedcomponent having a different phase; separating from said reflected lightsignal said first mixed reflected polarized light signal componenthaving a first phase and said second mixed reflected polarized lightsignal component having a different phase, wherein said first mixedreflected polarized light signal component comprises both P-polarizedand S-polarized light relative to the plane of incidence of saidreflected light signal, and wherein said second mixed reflectedpolarized light signal component comprises both P-polarized andS-polarized light relative to the plane of incidence of said reflectedlight signal; detecting a first intensity of said first mixed reflectedpolarized light signal component; detecting a second intensity of saidsecond mixed reflected polarized light signal component; determining adifference in phase between said first and second mixed reflectedpolarized light signal components based upon said first and secondintensities; and measuring at least one of the Kerr effect and thelubricant thickness based upon said difference in phase.